Jaguar Repair Problems? Ask a Jaguar Repairs Expert.
Hi there. Welcome to our site.
I'm a little confused on how specifically I can help you; can you clarify whether you're just seeking an educated guess on what could be wrong, or are you looking for technical guidance on how to fix the issue yourself – if the latter, must have tools and basic mechanical skills?
That's good to know because you'll need to do basically the same thing you would do if you were checking timing chain slack on a marine inboard V8; that is remove the distributor cap and mark the position of the rotor in relation to the housing, then have a helper slowly turn the engine by hand counterclockwise until the rotor just begins to move. At that point, have the helper turn the engine slowly clockwise until the rotor just begins to move once again. If it took turning the engine more than 4 degrees to get the rotor to start moving, the timing chain has too much slack. The slack can be caused by worn guides/shoes, weak/worn tensioner and/or stretched timing chain. When slack is found, it is recommended to replace all the timing components.
Replacing the timing chain on a 5.3 V12 takes any where from 33 to 40 hours.
The valve and ignition timing will be all over the place with a stretched chain. If you had high compression and big camshafts you would probably have damaged the pistons and valves by now. However, I'm not here to argue the point with you. As a certified ASE Master Technician and experienced Jaguar dealer technician who has worked on many of these V12's, I can only try to help you to the best of my ability and experience.
Don't go by just the mileage. I've seen timing chain tensioners fail on new engines still under warranty.
I know it could be a mistake to just go and tear the engine apart without first performing the appropriate diagnostic tests. Also, it is a mistake to listen to folks who offer guesses without the diagnostic procedure to substantiate their claims.
I hate wasting time, both yours and mine, refuting baseless arguments such as the one that claims that a throttle position sensor could cause timing to act as you describe. Why won't you do as I suggested? Think of it this way; once you have the distributor cap off, you could not only check timing chain slack, but also check the condition of the components in the distributor (see attached graphic).